1. Field of the Invention
The present invention relates to a method of controlling the elimination of roll eccentricity in a rolling mill of the type having backup rolls and a device for carrying out the method.
2. Description of Prior Art
In the rolling mill for the production of steel sheets or the like, the variations in thickness of the piece of metal as well as the variations in tension resulting from the variations in roll gap caused by the eccentricity of backup rolls adversely affect the quality of rolled products and the stable rolling operation.
Especially, the rolling mills provided with hydraulic screw-down mechanism having a fast response time have been recently used, but in order to obtain the rolled products having a high degree of accuracy of thickness by utilizing such fast response time, the eccentricity of the backup rolls must be completely eliminated.
Now let us consider a rolling mill comprising a pair of working rolls and a pair of backup rolls. In general, the motion of roll eccentricity contains not only a fundamental frequency but also harmonic frequency components. But, for the sake of simple explanation, let us consider only the fundamental frequency component whose period is equal to one rotation of each backup roll.
When the eccentricity of the top and bottom backup rolls is represented by .DELTA.S.sub.A and .DELTA.S.sub.B, respectively, the combined roll eccentricity .DELTA.S.sub.E is expressed by Eq. (1): EQU .DELTA.S.sub.E =.DELTA.S.sub.A +.DELTA.S.sub.B ( 1) EQU .DELTA.S.sub.A =X.sub.A .multidot.sin (.theta..sub.A +.PHI..sub.A) (2) EQU .DELTA.S.sub.B =X.sub.B .multidot.sin (.theta..sub.B +.PHI..sub.B) (3)
where
X.sub.A : an amplitude of eccentricity of the top backup roll; PA1 X.sub.B : an amplitude of eccentricity of the bottom backup roll; PA1 .theta..sub.A : an angle of rotation of the top backup roll; PA1 .theta..sub.B : an angle of rotation of the bottom backup roll; PA1 .PHI..sub.A : a phase of the top backup roll when .theta..sub.A =0; and PA1 .PHI..sub.B : a phase of the bottom backup roll when .theta..sub.B =0.
In general, the degree of roll eccentricity is detected in response to the combined eccentricity .DELTA.S.sub.E of the top and bottom backup rolls detected from the rolling load signal.
Recently, there has been devised and demonstrated a method in which, in order to control the crown or the shapes of rolled metal pieces, the pheripheral speeds of the upper and lower working rolls are respectively varied. In this method, however, the top and bottom backup rolls are different in eccentricity frequency from each other so that the degrees of eccentricity of the top and bottom backup rolls must be detected separately and then their eccentricity must be eliminated. Furthermore, even when the peripheral speed of the upper and lower working rolls are same, when the top and bottom backup rolls are different in diameter from each other, they are different in eccentricity frequency from each other.
For instance, the method for separately detecting the eccentricity of the top and bottom backup rolls is disclosed in detail in Japanese Patent Publication No. 56-22281 or Japanese Patent Application Laid-Open No. 60-141321. According to this method, the screw-down is carried out in the so-called kiss roll mode; that is, in the mode in which no piece of metal is rolled, so that some great load is produced and the load signal is subjected to the Fourier transformation by using the rotational speeds and load signals of the top and bottom backup rolls, whereby the eccentricity of the top and bottom backup rolls can be separately detected.
In response to the angles of rotation of the top and bottom backup rolls, respectively, the amplitude of eccentricity thus detected are reproduced and the reproduced signals are applied as the reference signals to the roll gap control device in the direction in which the variations in roll gap due to the roll eccentricity can be eliminated, so that the variations in roll gap due to the roll eccentricity can be eliminated and consequently the thickness of the rolled product can be controlled with a high degree of accuracy. It follows therefore that when the eccentricity detected in the so-called kiss roll state is equal to that detected during the rolling operation, the control for eliminating the roll eccentricity can be carried out with a high degree of accuracy.
It is well known to those skilled in the art that there exists the variation in roll eccentricity or the aging of the roll eccentricity depending upon the magnitude of the rolling load so that under various rolling conditions, it is almost impossible to detect the roll eccentricity with a high degree of accuracy by the method described above.
Furthermore, in the case of the completely continuously type rolling mill in which the pieces of metal are successively welded into a continuous piece which in turn is continuously rolled without stopping the rolling operation of the rolling mill, the chance for obtaining the so-called kiss roll state is less so that the application of the above-described method is difficult.